Abstract:

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The present work deals with the relationship between microstructure and mechanical properties of A356 aluminum alloy which was produced via thixocasting process under different casting conditions. Feedstock billets were heated to a target temperature to obtain a semi-solid slurry with the required solid fraction. Some billets were heated to a fully-melted condition. In order to obtain fine and spheroidized Al grains, some billets for the partially melting were compressed axially by 33% at a room temperature before heating. The completely-melted and partially-melted slurries were die-cast by using a die-cast machine, and hour glass-shaped rod-type tensile specimens and small-size plate-type tensile specimens were obtained. Small cubic specimens were also collected from the die-cast products for microstructural evaluation. They were polished, and etched by Weck’s reagent. The partially-melted specimen which was compressed before heating shows the spherical Al grains. But the grain of the strain-free partially-melted specimen exhibited complicated morphology. The fully-melted specimen shows the fine and dendrite structure.

Abstract: By using equal channel angular extrusion (ECAE) as strain induced step in strain induced
melt activated (SIMA) and completing melt activated step by using semi-solid isothermal treatment, a
new SIMA method is introduced firstly. The results show that semi-solid billet with highly spheroidal
and homogeneous grains with the average grain size of 20μm can be prepared by new SIMA method.
High mechanical properties, such as ultimate tensile strength of 321.8MPa and elongation of 15.2%
are obtained in magazine plate components thixoforged using semi-solid billet prepared by new
SIMA.

Abstract: Preparation of semi-solid microstructure of 7075 aluminum alloy industrial extrusion
billets was studied in this paper. A new semi-solid microstructure preparation process is proposed. In
the treatment, melting-stirring and predeformation of the alloy billets are not required. An ideal
microstructure and higher dimensional precision of the billet can be obtained only with a direct
heating and insulation method. The influences of different heating temperatures and insulation time
on the microstructure evolution were studied with orthogonal testing methods. The obtained
microstructure was observed and analyzed by optical microscopy, and the formation mechanism of
the semi-solid microstructure is further discussed. The results indicate that a fine microstructure can
be obtained with the proposed process and the processing parameters can be controlled over a wide
range. Also, the grain microstructure obtained by the present process is better than that of the SIMA.
For 7075 aluminum alloy billets, perfect fine equiaxial grains can be obtained under a heating
temperature of 620°C and a holding time of about 25 minutes. The average grain size is around 80μm.

Abstract: In semi-solid remelting process, the various stages of reheating temperature and isothermal holding time must be accurately controlled in order to obtain the uniformly distributed and small equiaxed grains microstructure. In this paper, a temperature control program was developed and the remelting process for Al-7Si-2RE aluminum alloy was carried out. The results showed that with the raise of reheating temperature and the extension of isothermal holding time, the liquid fraction increases, α-phase grain grows and becomes rounding in the process of Al-7Si-2RE alloy semi-solid remelting. The most reasonable process parameter of reheating temperature is at 585~590°C and its appropriate isothermal holding time are about 10~15min for the semi-solid Al-7Si-2RE alloy.

Abstract: Stress-induced melt activation (SIMA) has a certain advantage during the preparation of semi-solid billet of Mg alloy. However, Equal-Channel Angular Pressing (ECAP) only can produce small billet by several channels to achieve large equivalent strain. To conquer this problem, the forward extrusion/(ECAP) method, a new process for preparing semi-solid Mg alloy was proposed. And the feasibility of forward extrusion/ECAP was analyzed. This new method simplified the processing procedure for providing commercial semisolid billets of Mg alloy. The experimental dies and experimental procedure were designed. The effect of extrusion ratio and equivalent strain on the microstructure of semisolid Mg alloy was investigated. It was found that when the extrusion ratio was larger than 10, the semisolid microstructure is refined, with increasing extrusion ratio, the microstructure is refined further.

Abstract: A356 aluminium alloy has a wide applicability in the manufacturing of automotive parts. Cooling slope (CS) rheocasting process has been used in the present work to produce A356 billets having near spherical morphology of primary Al phase. Absence of dendritic primary phase, observed in case conventionally cast A356 alloy, and finer distribution of secondary eutectic Si phase within the matrix establishes the usefulness of the CS casting route. Near spherical primary phase in the rheocast alloy ensures better strength, elongation properties and structural integrity in the produced billets. The liquid melt is allowed to flow through the cooling slope after pouring at 6500C. Rapid heat exchange between the flowing melt and slope wall and the atmosphere facilitates heterogeneous nucleation of α-Al phase on the cooling slope wall. Shear driven flow of the solidifying melt is found responsible for separation of α-Al phase from the slope wall and generation of nearly spherical morphology of the primary phase in the microstructure. Grain refiner addition in the melt leads to enhance the primary α-Al percentage in the microstructure and also aids to the improvement of degree of sphericity and reduction of spheroid size. So, grain refining helps to improve the strength, elongation and fracture properties of rheocast billets further.